1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device in which a reactive gas is thermally decomposed to form a film on a semiconductor substrate, in particular, a method of forming a film by using a CVD (chemical vapor deposition) apparatus.
2. Description of the Related Art
In the manufacture of semiconductor devices including ICs and LSIs, a conductive film such as a PolySi (poly silicon) film and a W (tungsten) film is formed by using LPCVD (low pressure chemical vapor deposition) method.
FIG. 1 shows a schematic view of a LPCVD apparatus for forming a PolySi film.
Fifty to a hundred and fifty semiconductor wafers 11 are set on a boat 12, and introduced into a quartz tube 13. The quartz tube 13 includes therein an inner quartz tube 113. A heater 14 for heating the semiconductor wafers 11 is provided outside the quartz tube 13 and generates a radiant heat. The radiant heat of the heater 14 passes the wall of the outer quartz tube 13, the wall of the inner quartz tube 113, and then reaches and heats semiconductor wafers 11, so that the semiconductor wafers 11 are heated to a temperature of 600° C. A temperature near the heater 14 is measured by the outside thermocouple 111 provided outside the quartz tube 13, that is, between the heater 14 and the quartz tube 13 in detail, whereas a temperature near the semiconductor wafers 11 is measured by the inside thermocouple 112 provided inside the quartz tube 13.
The inside of the quartz tube 13 is evacuated by a pump 15 to reduce the pressure in the inside of the quartz tube 13, and an SiH4 gas as a film forming gas is introduced into the quartz tube 13 via a nozzle 16, a mass flow controller 17 and a valve 18. The gas, after having been introduced into the quartz tube 13, passes through the inside of the inner quartz tube 113, and then is discharged through a gap between the inner quartz tube 113 and the quartz tube 13. The pressure inside the quartz tube 13 is maintained at 0.5 Torr by the conductance control of a main valve 110, based on an indication value of a pressure gage 19. The temperature of the semiconductor wafers 11 is controlled on the basis of a temperature measured by outer thermocouples 111 provided outside the quartz tube 13, i.e., between the heater 14 and the quartz tube 13 and inner thermocouples 112 provided inside the quartz tube 13. With the above arrangement, the SiH4 introduced in the quartz tube 13 thermally is decomposed in the quartz tube 13, thus depositing PolySi on the semiconductor wafers 11. In this example, the outer thermocouples 111 are provided at three measuring points and similarly the inner thermocouples 112 are provided at three measuring points.
In this time, not only the temperature of the semiconductor wafers 11 but also the temperature of the wall of the quartz tube 13 are increased, thus depositing PolySi on the inner surface of the wall of the quartz tube 13. However, the temperature of the wall of the quartz tube 13 cannot be accurately measured: because quartz is semi-transparent with respect to radiant heat of the heater 14. For this reason, it is impossible to precisely determine how thick PolySi has been deposited on the inner surface of the wall of the quartz tube 13. Further, PolySi film having a thickness less than 1 μm is semi-transparent and allows infrared rays to pass therethrough. Therefore, as the thickness of the PolySi film on the inner surface of the wall of the quartz tube 13 is increased, then an amount of radiant heat that is received by the semiconductor wafers 11 changes, and thus the temperature of the semiconductor wafers 11 changes accordingly. Hence, in this state, it is impossible to manufacture PolySi films of a uniform thickness.
Conventionally, in order to avoid this problem, the PolySi deposited on the inner surface of the wall of the quartz tube 13 is removed by etching with gas or liquid of acid. Then, a PolySi film is deposited on the inner surface of the wall of the quartz tube 13 to a thickness of 0.3 μm or more, and typically to about 0.5 μm. Thereafter, a test film-formation is performed in order to confirm that the film forming conditions are proper. After that, the manufacturing process is performed. Since the conventional method needs the additional step of the PolySi film deposition, the operation rate of the manufacturing facility is low.
As described above, in the conventional method of manufacturing a semiconductor device, in order to assure the stability of the film formation, there must be added a step of depositing a PolySi film having a predetermined thickness in advance in the inner surface of the wall of the quartz tube 13 so that the thickness of the PolySi film deposited in the inner surface of the wall of the quartz tube 13 will increase to 1 μm or more during the manufacture process. This additional step decreases the operation rate of the apparatus.